Mobile telephony networks have experienced an enormous spread, especially after the introduction of Second Generation (2G) mobile cellular networks, and particularly digital mobile cellular networks such as those complying with the Global System for Mobile communications (GSM) standard (and its United States and Japanese counterparts). The services offered by these cellular networks in addition to plain voice communications have rapidly increased in number and quality; just to cite a few examples, Short Messaging System (SMS), Multimedia Messaging System (MMS) services, and Internet connectivity services have been made available to subscribers of the cellular network in the last few years. It is expected that further services will be progressively made available to subscribers, especially with the advent of the Third Generation (3G) or Fourth Generation (4G) mobile networks.
A conventional GSM network 1, referred to as a public land mobile network (PLMN), is schematically illustrated in FIG. 1. A mobile switching center (MSC) 2 is connected via communication links to a number of base station controllers (BSCs) 4. The BSCs 4 are geographically dispersed across areas served by the mobile switching center 2. Each BSC controls one or more base transceiver stations (BTSs) 6 located remote from, and connected by further communication links to, its associated BSC. Each BTS 6 transmits radio signals to, and receives radio signals from, mobile stations 8 which are in an area served by that BTS. That area is generally referred to as a “cell”. A GSM network is typically provided with a large number of such cells, which are ideally contiguous to provide continuous coverage over the whole network territory.
A mobile switching center 2 may be also connected via communications links to other mobile switching centers in the remainder of the mobile communications network 10, and to a public service telephone network (PSTN), which is not illustrated in FIG. 1. The mobile switching center 2 is associated with a home location register (HLR) 12, which is a database storing subscriber authentication data including the international mobile subscriber identity (IMSI) which is unique to each mobile station 8. The IMSI is also stored in the mobile station in a subscriber identity module (SIM) along with other subscriber-specific information.
The IMSI is a string of digits, including a mobile country code, typically indicating the home country of the mobile, a home public land mobile network (PLAN) code, indicating the home mobile network of the mobile station, an HLR code indicating the identity of the HLR in which the subscriber information is stored, and a subscriber identity number identifying the subscriber in the HLR.
The mobile switching center 2 is also associated with a visitor location register (VLR) 14, which is a database temporarily storing subscriber authentication data including the temporary mobile subscriber identity (TMSI) which uniquely identifies a mobile station in the VLR, for mobile stations which are active in the area served by the mobile switching center 2. Before subscriber authentication data is entered in a VLR, the subscriber is authenticated by accessing the subscriber's HLR.
Network sharing is a known technique that allows some physical components of a mobile network to be shared by different network operators. For example, White Paper titled “Shared Networks for WCDMA”, by Ericsson (August 2003), available at the Internet address http://www.ericsson.com/products/white papers pdf/251544_SharedNetw_WP_Final.pdf discloses several solutions for infrastructure sharing among operators, especially for 3G business. Site Sharing typically involves sharing the site and mast for antenna placement. In addition to this, the power equipment, transmission equipment and antennas can be shared among operators. In the Shared UTRAN (UMTS Terrestrial Radio Access Network) the radio network is shared. One radio base station (RBS) is deployed per site, with individual frequencies for each operator. The radio network consists of RBS, Radio Network Controller (RNC) and transmission. The nodes are physically shared. The sharing operators deploy their individual core network. In the Common Shared Network solution the operators jointly build and operate one common 3G network, consisting of the radio network and part of the core network. The operators share the RBS, RNC, Mobile Services Switching Center/Visitor Location Register (MSC/VLR) and Serving GPRS Support Node (SGSN). Each operator has its individual home network that contains the independent subscriber databases, services, subscriber billing and connection with external networks. The operators have individual Home Location Register (HLR), Gateway GPRS Support Node (GGSN), GMSC and service platforms.
The patent application WO 98/02008 discloses a public mobile communications network configured according to an established protocol (such as GSM) which includes a private network portion to which only a selected set of mobile stations have access. The network is modified by the provision of one or more additional base transceiver stations and a set of private subscriber mobile stations which use a private PLMN code which is different to the public PLMN code of the main network. Each of the private subscriber mobile stations has the private PLMN code stored in its SIM as its home PLMN code. The private PLMN code forms part of its subscriber identity (IMSI) as prescribed by the GSM protocol. In addition, the SIM stores in a memory field a preferred PLMN list. The public PLMN code is given the status of most preferred alternative to the private PLMN code. The additional BTS, which functions compatibly with the main network BSCs, is controlled by its serving BSC to broadcast the private PLMN code, rather than the public PLMN code (which is broadcast by the public network portion BTSs), on its broadcast control channel (BCCH). The additional BTS may for example be located inside a building to provide in-building coverage. The MSC is modified to enable it to receive and respond to a service request originating from a private subscriber mobile station, transmitted via the additional BTS. The home location register is modified to store subscriber authentication data relating not only to the public subscriber mobile stations, but also to the private subscriber mobile stations. The additional network portion provided may be a network portion in which a modified set of services or call tariffs is available, possibly only to a select group of mobile stations using a network identity code which is different to that used by the remainder of the mobile stations.